Numerous research areas in biochemistry depend on the ability to analyze minute quantities of nucleic acids, amino acids, and peptides, yet many present detection schemes have inadequate sensitivity. Examples are the analyses of single cells and subcellular compartments. One of the few techniques to permit successful assays of the contents of a single cell is capillary zone electrophoresis (CZE), a powerful separation technique for the analysis of small sample volumes. Olefirowicz, T. M.; Ewing, A. G., Anal. Chem., 1990, 62, 1872-1876; Wallingford, R. A.; Ewing, A. G., Anal. Chem., 1988, 60, 1972-1975; Chien J. B.; Wallingford, R. A.; Ewing, A. G., J. Neurochem., 1990, 54, 633-638; and Kennedy, R. T.; Oates, M. D.; Cooper, B. R.; Nickerson, B.; Jorgenson, J. W., Science, 1989, 246, 57-63. Separation efficiencies routinely exceed several hundred thousand theoretical plates, and typical injection volumes are 10 nL or less.
For CZE, channel diameters usually range from 25 to 100 microns; therefore, designing methods to detect low concentrations in these small capillaries is a challenge. Laser-induced fluorescence (LIF) is currently the most sensitive detection method for CZE; detection limits are in the low attomole range. Nickerson, B.; Jorgenson, J. W., , J. High Resolut. Chromatogr. Commun., 1988, 11, 533-534; Drossman, H.; Luckey, J. A.; Kastichka, A. J.; D'Cunhan, J.; Smith, L. M., Anal. Chem., 1990, 62, 900-903; and Kuhr, W. G.; and Yeung, E. S., Anal. Chem., 1988, 60, 2642-2646. In these systems, the capillary is used as the flow cell, the laser illumination is perpendicular to the capillary, and a photomultiplier tube (PMT) monitors the fluorescence. Dovichi and coworkers developed a more sensitive method that has the same excitation geometry but uses a sheath flow cuvette as the sample cell and thereby eliminates much of the scattered light and luminescence from the fused silica capillary. With this technique, detection limits in the low zeptomole range (10.sup.-21 moles) for fluorescently tagged amino acids have been reported. Cheng, Y.-F.; Dovichi, N. J.; Science, 1988, 242, 562-564; Wu, S.; Dovichi, N. J.; J. Chromatogr., 1989, 480, 141-155; Dovichi, N. J.; and Cheng, Y.-F., Am. Biotechnol. Lab., 1989, 7, 10-14.
However, despite the above advances in detection techniques, there remains a need for detection methods and devices with greater sensitivity.